This invention relates generally to a ThO.sub.2 catalyst, and more particularly, to a ThO.sub.2 catalyst having a high surface area.
ThO.sub.2 has found widespread use as a catalyst, particularly for the catalysis of hydrocarbon synthesis.
Previously, ThO.sub.2 has been synthesized by the decomposition of Th nitrate, oxalate and hydroxide (Morehead, D. R., McCartney, E. R., J. Aust. Ceram. Soc., 12, 1977, pp. 27-33). In this method of synthesis, the respective compounds are thermally decomposed in flowing air and nitrogen, and under vacuum conditions. This method of synthesis produces ThO.sub.2 compounds having the following surface areas:
TABLE I ______________________________________ Surface area of ThO.sub.2 (m.sup.2 /g) ______________________________________ Thorium Oxalate 50 Thorium Nitrate 44 Thorium Hydroxide 39 ______________________________________
In another synthesis method, thorium hydroxide is precipitated from a solution of thorium nitrate by the addition of ammonia, followed by filtering, washing, and drying of the precipitate, and inactivation by heating at a suitable elevated temperature between about 400.degree. and 800.degree. C. (Brey, W. S.; Davis, B. H.; Schmidt, P. G.; and Moreland, C. G.; J. Cat. 3, 1964, pp 303-311). This work discloses that both the conditions of the precipitation of the hydroxide and the conditions of conversion of the hydroxide to the oxide affect the properties of the final product. It also discloses that two of the qualitative circumstances surrounding the formation of the hydroxide from which the oxide is derived were found to be significant. It was discovered that the rate at which ammonia was added to the solution of thorium nitrate contributed to the ThO.sub.2 surface area. In this regard, a rapid rise of surface area of ThO.sub.2 was observed during the early washing stages, followed by a maximum value beyond which there was a plateau or slow decline with further washing.
The time of heating of the material during the preparation of the oxide was also found to affect the oxide surface area. The longer the time of heating, the smaller the surface area. However, this method yields a ThO.sub.2 catalyst having a surface area of no more than about 50 m.sup.2 /g.
A third method of ThO.sub.2 synthesis is disclosed in Breysse, M., Ann. Chem., 2 (1967), pp 367-389. In this synthesis method, ThO.sub.2 is produced by calcination of the oxalate. This is achieved by precipitating the salt by the action of oxalic acid on a solution of thorium nitrate. The investigators concluded that the surface area of ThO.sub.2 increases with: the initial pH of the thorium nitrate solution; the concentration of thorium nitrate; time to mix the reactants; precipitation temperature; and time of aging. A complete list of the parameter study, parameter variations, and surface area of ThO.sub.2 is listed in Table II.
TABLE II ______________________________________ ThO.sub.2 spent Parameter Studied Parameter Variation area, m.sup.2 /g ______________________________________ Initial pH of 2.2 33 nitrate solution 0.7 36 Rate of addition 50 ml in 20 min. 12 50 ml in 60 min. 25 Order of addition acid into nitrate 40 nitrate into acid 34 Concentration of M 40 thorium nitrate M/10 27 solution M/50 18 Concentration 0 28 ratio: excess 20% 24 acid 100% 19 Precipitation 34.degree. C. 17 temperature 75.degree. C. 24 boiling 33 Aging of 0 24 precipitate 1 hour.sup. 28 4 hours 30 ______________________________________
Interestingly, the investigators disclosed that the slower the rate of addition of the reactants, the greater the surface area. The largest ThO.sub.2 surface area obtained by varying the parameters was about 40 m.sup.2 /g.
It would be an advancement in the art to provide a method for synthesizing a ThO.sub.2 catalyst having a surface area of at least 100 m.sup.2 /g. Such a catalyst would provide powerful catalytic behavior never before attained.